In many connections it is desirable to be able to determine an absolute position on a surface. One example is the digitizing of drawings. Another example is the production of an electronic version of handwritten information.
Examples of previously known devices for position determination are found in U.S. Pat. No. 5,852,434, where a device for determining an absolute position is described. The device comprises a writing surface which is provided with a position-coding pattern by means of which x-y-coordinates can be determined, a detector that can detect the position-coding pattern and a processor that can determine the position of the detector in relation to the writing surface on the basis of the detected position-coding pattern. The device makes it possible for a user to enter handwritten and hand-drawn information into a computer while the information is being written or drawn on the writing surface.
Three examples of position coding are given in U.S. Pat. No. 5,852,434. The first example consists of symbols, each of which is constructed of three concentric circles. The outermost circle represents the x-coordinate and the middle the y-coordinate. Both the outer circles are further divided into 16 parts which, depending upon whether they are filled in or not, indicate different numbers. This means that each pair of coordinates, x, y, is coded by a complex symbol with a special appearance.
In the second example, the coordinates in each point on the writing surface are indicated by means of a bar code stack, a bar code for the x-coordinate being indicated above a bar code for the y-coordinate.
A third example consists of a checked pattern that can be used to code the x- and y-coordinates. There is, however, no explanation as to how the checked pattern is constructed or how it can be converted into coordinates.
A problem with the position-coding pattern of U.S. Pat. No. 5,852,434 is that it is constructed of complex symbols and the smaller the symbols are made, the more difficult it becomes to produce the patterned writing surface and the greater the danger of incorrect position determinations, while the larger the symbols are made, the poorer the position resolution becomes.
A further problem is that the processing of the detected position-coding pattern by the processor becomes rather complicated, due to the fact that complex symbols are to be interpreted.
Yet another problem is that the sensor must be designed in such a way that it can record four symbols at the same time, so that it is certain to include at least one symbol in its entirety, which is necessary in order for the position determination to be carried out. The ratio between the required sensor area and the area of the position-coding pattern that defines a position is thus large.
In the international Patent Application PCT/SE00/01895, which is assigned to the present Applicant, a position code is described which solves the above-mentioned problems. The position code consists of a raster and marks which are positioned at each raster point. The marks are preferably essentially the same size, round and displaced in relation to the raster points in one of four orthogonal directions. The raster is virtual and is thus invisible both to the eye and to sensors.
In order to decode the above-mentioned position code, it is necessary for the virtual raster to be identified. The identification of the raster is the object of the present invention.